If you are inside 30 days, you may return for a refund (Call Adobe)
If outside 30 days... you should still call Adobe to find out if they will give you an install # for CS3
I don't use Cineform, so this is merely conjecture, but isn't there a new-ish version of the codecs/plugins that is CS4-compatible? I seem to remember others posting that older versions of Cineform's products wouldn't work in CS4, and that an update was necessary (again, I didn't follow too closely, not being a Cineform user). Or is it the new version that's giving you problems? If so, you might want to contact Cineform, as CS4--especially with the last update--has been pretty stable and usable.
There was talk that Cineform was working on a CS4.1 version. I just do not recall hearing whether it has been completed yet.
As you suggest, contacting Cineform will likely get updated info and maybe a cure to the OP's issues.